Add detection of python3 binary.

[nirvdrum]

Python 2 is being or has been phased out of default installations on many systems. PEP 394 means systems such as Ubuntu will not alias python3 to python by default. In order to use the Python 3 binary with mx, the end user must either set MX_PYTHON=python3, MX_PYTHON_VERSION=3, or set up an alias by some other means. It would be helpful if mx checked if python3 exists and use it, as part of its existing executable search.

This should maintain full backwards-compatibility. The python3 binary is only chosen if python2.7, python2, and python do not exist.

[zakkak]

I think python3 should be preferred when available even if that breaks "full backwards-compatibility" as python2 has reached its end of life.

[nirvdrum]

@dougxc @gilles-duboscq Can we favor Python 3 over 2? If so, I'll reorder the executable checks in this PR.

Additionally, that Python 3.x gate failure looks unrelated to my changes.

[fniephaus]

Can we favor Python 3 over 2?

IIRC we looked into exactly this only recently and ran into a bunch of problems that need to be addressed first (or simultaneously). Anyway, I agree that it needs to happen sooner rather than later.

[fniephaus]

Assigning to @dougxc who is currently working on this. We may be dropping Python 2 support entirely soon.

[nirvdrum]

Thanks. Regardless of dropping Python 2 support, this PR would help environments where only Python 3 is installed.

[dougxc]

I agree that merging this in the interim is a good idea. Would you be able to take care of that @fniephaus while I focus on the larger change to drop python2 altogether?

BTW, I've opened #249 to track the larger change.